1. Field of Inventions
The present inventions relate generally to devices for performing therapeutic operations on body tissue.
2. Description of the Related Art
There are many instances where therapeutic elements must be positioned adjacent to body tissue. One instance involves the formation of therapeutic lesions to the treat cardiac conditions such as atrial fibrillation, atrial flutter and arrhythmia. Therapeutic lesions, which may also be used to treat conditions in other regions of the body such as the prostate, liver, brain, gall bladder, uterus, breasts, lungs and other solid organs, are typically formed by ablating tissue.
Cryogenic cooling devices are one example of the devices that have been used to form lesions in tissue. During the cryo-ablation of soft tissue (i.e. tissue other than blood, bone and connective tissue), ice crystals disrupt cell and organelle membranes and it is the disruption that kills the tissue. A cryogenic element, such as a balloon or hollow metal tip, is carried on the distal end of a catheter or surgical probe (referred to herein collectively as “probes”), placed in contact with tissue and cooled to a temperature that will cause tissue death. The cryogenic element may be cooled by a variety of techniques. One technique employs the Joule-Thompson (“JT”) effect. Here, cryogenic cooling occurs as a result of a rapid decrease of gas pressure that occurs within the therapeutic element. Pressurized cryogenic fluid, such as liquid nitrous oxide, is directed into the therapeutic element where it undergoes rapid phase change and a rapid expansion of the gas from a high-pressure to a lower pressure state. The reaction is endothermic and produces temperatures as low as minus 70.degree. C. at the therapeutic element. In some instances, the cryogenic fluid is pre-cooled in order to increase the cooling power delivered to the targeted tissue. The cryogenic element may also be cooled by directing super-cooled fluid through the catheter or surgical probe to the cryogenic element. Here, the temperature at the therapeutic element can be as low as minus 100.degree. C. when it enters the patient.
The present inventors have determined that conventional cryogenic cooling devices are susceptible to improvement. For example, the present inventors have determined that conventional cryogenic cooling devices can damage non-target tissue near the tissue in which the therapeutic lesions are being formed. The present inventors have also determined that it can be difficult to achieve good tissue contact with conventional cryogenic devices because they are relatively turgid. Some conventional cryogenic cooling devices are susceptible to leaks, which can result in the release of toxic chemicals (e.g. perfluorocarbons) into the patient's blood stream during endocardial procedures. Additionally, the inventors herein have determined that the manner in which temperature is monitored during cryogenic lesion formation procedures.